The present invention relates to a high voltage supply circuit and method of supplying a high voltage, and more particularly, to a high voltage supply circuit and method of supplying a high voltage, capable of preventing a pumping voltage from being lowered than a target voltage.
In general, a unit cell of a DRAM consists of one MOS type transistor serving as a switch and one capacitor for storing a charge (data). A plurality of the unit cells are classified into banks. At this time, the unit cell has a binary data stored therein. Depending on a charge state stored in the capacitor included in the unit cell, the data stored in the unit cell is divided into ‘1’ or ‘0’. As such, as the data is decided depending on the charge stored in the capacitor, there is no power consumption in principle.
As the leakage current occurs in a PN junction of the MOS transistor, etc., however, the amount of the charge stored in the capacitor is gradually reduced as time goes by. For this reason, there is a high possibility that the data may be lost. Therefore, in order to prevent the data from being lost, it is required that after reading a data of a memory cell, the charge be recharged again into the capacitor depending on its information. Such operation is called ‘a refresh operation’. It is required that the refresh operation be periodically repeated so that the data is not lost. A voltage that is applied to the unit cell for the refresh operation is hereinafter referred to as ‘a refresh voltage’. As the refresh voltage is higher than the power supply voltage, a pumping voltage being a result that the power supply voltage is raised over a given voltage is used as the refresh voltage.
FIG. 1 is a block diagram illustrating a conventional high voltage supply circuit.
Referring to FIG. 1, the high voltage supply circuit for generating a high voltage such as the refresh voltage used in a DRAM includes a pumping voltage detector 110, an oscillator 120 and a pumping unit 130.
If the entire circuit is initialized and then normally starts to operate, an active signal is generated. The active signal is applied to the pumping voltage detector 110 as an enable signal E1. Based on the enable signal, the pumping voltage detector 110 determines the potential of a pumping voltage (Vpp) generated in the pumping unit 130. The oscillator 120 generates a pulse signal according to the output signal of the pumping voltage detector 110. The pumping unit 130 raises the power supply voltage (Vdd) using the pulse signal generated in the oscillator 120, to generate the pumping voltage (Vpp). The pumping unit 130 then supplies the raised pumping voltage (Vpp) to a peripheral circuit 140. At this time, the pumping voltage detector 110 determines the potential of the pumping voltage (Vpp) generated in the pumping unit 130. As a result of the determination, if the pumping voltage (Vpp) is lower than a target voltage, the pumping voltage detector 110 operates the oscillator 120 to consistently raise the power supply voltage (Vdd) until the pumping voltage (Vpp) becomes equal to the target voltage. If the pumping voltage (Vpp) becomes equal to the target voltage, the pumping voltage detector 110 stops the operation of the oscillator 120 so that the pumping operation is stopped.
Meanwhile, the cells included in the DRAM are classified into the banks. In a normal operating mode, only one bank is enabled or a plurality of the banks are sequentially enabled. In this case, since the word lines, etc. are sequentially selected, the peak current depending on the pumping voltage (Vpp) is adequately distributed and supplied.
If the normal operating mode is switched to an auto-refresh operating mode, however, the plurality of the banks are enabled at the same time. Due to this, the pumping voltage (Vpp) is lowered while the pumping voltage (Vpp) is applied to all the banks. At this time, the pumping voltage detector 110 operates the oscillator 120 so that the pumping operation is performed when the pumping voltage (Vpp) becomes lower than the target voltage. If the oscillator 120 is operated to generate the pulse, the pumping unit 130 performs the pumping operation to raise the pumping voltage (Vpp) up to the target voltage again.
As such, the conventional high voltage supply circuit can always keep the pumping voltage (Vpp) equal to the target voltage. However, if the normal mode is switched to an operating mode where current consumption is abruptly increased such as the auto-refresh operation, there is a problem that the pumping voltage (Vpp) is lower than the target voltage and is then increased up to the target voltage. In other words, until the pumping voltage detector 110 senses the reduced pumping voltage (Vpp) and then performs the pumping operation to raise the pumping voltage (Vpp) up to the target voltage again, the circuit is operated in a state where the pumping voltage (Vpp) is lower than the target voltage. The circuit may be thus erroneously operated in this period. Accordingly, there is a problem that reliability in the operation of the circuit is degraded.